Memory cell with schottky diode

ABSTRACT

Memory cell comprising two conductors, with a serially connected magnetic storage element and a Schottky diode between the two conductors. The Schottky diode provides a unidirectional conductive path between the two conductors and through the element. The Schottky diode is formed between a metal layer in one of the two conductors and a processed junction layer. Methods for process and for operation of the memory cell are also disclosed. The memory cell using the Schottky diode can be designed for high speed operation and with high density of integration. Advantageously, the junction layer can also be used as a hard mask for defining the individual magnetic storage element in the memory cell. The memory cell is particularly useful for magnetic random access memory (MRAM) circuits.

FIELD

Embodiments of the invention relate to magnetic memory circuits, and inparticular, to magnetic random access memory (MRAM) circuits.

BACKGROUND

Magnetic memory circuits are based on magneto-resistive behavior ofmagnetic storage elements that are integrated typically with acomplementary metal-oxide-semiconductor (CMOS) technology. Such memorycircuits generally provide non-volatility and an unlimited read andwrite capability. An example is the magnetic random access memory (MRAM)circuit that includes a plurality of memory cells, each defining anaddressable magnetic storage element that may include a magnetic tunneljunction (MTJ) stack.

Each addressable MTJ stack can have a magnetic spin orientation and canbe flipped between two states by the application of a magnetic fieldthat is induced by energizing corresponding bit and word lines.

FIG. 1A illustrates a plan view of a section of an exemplary array 100of memory cells X 112 in a magnetic random access memory (MRAM) circuit,that includes a set of longitudinal word lines (WL) 102 and a set oftransverse bit lines (BL) 104. The set of BL 104 overlies the set of WL102 to define crossover zones 108. An addressable MTJ stack 110 isdisposed within each crossover zone 108. Current drivers 106 areprovided for energizing the BL 104 and the WL 102. An address transistor(not shown) is provided under each MTJ stack 110 and in the memory cellX 112, for reading the state of the MTJ stack 110.

FIG. 1B illustrates a partly schematic and partly cross-sectional viewof the memory cell X 112 in FIG. 1A. As shown in the cross-sectionalview, the MTJ stack 110 is disposed within the crossover zone 108. Theaddress transistor 132 is shown schematically. Generally, the MTJ stack110 is designed to be integrated into a back-end metallization structurefollowing a front-end CMOS processing. The MTJ stack 110 is shown to beprovided between a first metallization layer Mx and a secondmetallization layer My, wherein the MTJ stack 110 is connected to thefirst layer Mx through a via hole 128 and to the second layer My througha via hole 116. The second layer My is patterned to include the BL 104.The MTJ stack 110 includes a free layer 118, a tunnel oxide layer 120, afixed layer 122 and an extended bottom electrode 124. The first layer Mxis patterned to include the WL 102 for writing into the MTJ stack 110.The address transistor 132 is connected to the first layer Mx by aconnection 130 a. A read word line (WL) 130 b in the first layer Mx isusable for selectively operating the address transistor 132. The WL 102has no contact with the bottom electrode 124, and when energized,induces a magnetic field within the MTJ stack 110.

A write operation in a selected memory cell X 112 in the array 100 canbe performed by energizing the corresponding BL 104 and the WL 102, togenerate a magnetic field for changing the magnetic state of thecorresponding MTJ stack 110. For a read operation, a voltage is appliedto the BL 104 of the selected memory cell X 112, so that a current canflow through the corresponding MTJ stack 110 and the address transistor132 that is selectively switched on by the WL 130 b. The magnitude ofthe current sensed indicates the conductivity or the magnetic state ofthe MTJ stack 110.

SUMMARY OF THE INVENTION

According to an embodiment of a first aspect of the invention, anaddressable memory cell is proposed, that comprises two conductors witha serially connected magnetic storage element and a Schottky diodebetween the two conductors. The Schottky diode provides a unidirectionalconductive path between the two conductors and through the element. Thediode is disposed according to one of the following two configurations:

-   -   (i) between the first conductor and the element in a first        configuration, wherein the diode comprises a junction between a        first metal layer and a first junction layer, wherein the first        conductor comprises the first metal layer, and    -   (ii) between the second conductor and the element in a second        configuration, wherein the diode comprises a junction between a        second metal layer and a second junction layer, wherein the        second conductor comprises the second metal layer.

According to another embodiment of the first aspect of the invention,the first and second junction layers comprise titanium nitride (TiN).

According to another embodiment of the first aspect of the invention,the element is a magnetic tunnel junction (MTJ) stack.

According to another embodiment of the first aspect of the invention,the first conductor is a word line and the second conductor is a bitline.

According to another embodiment of the first aspect of the invention,the first metal layer and the second metal layer comprise aluminum.

According to another embodiment of the first aspect of the invention,the first conductor, the element, the diode and the second conductor areall vertically stacked on a substrate.

According to another embodiment of the first aspect of the invention,the second junction layer is atop the element in the secondconfiguration.

According to an embodiment of a second aspect of the invention, a methodof addressing a memory cell in a magnetic random access memory (MRAM)circuit is proposed, wherein the cell comprises a word line, a bit lineand a magnetic tunnel junction (MTJ) stack between the two lines. Themethod is to provide a unidirectional conductive path between the twolines and through the MTJ stack by using a Schottky diode that isserially connected to the MTJ stack. The diode is disposed according toone of the aforesaid two configurations.

According to embodiments of the first and second aspects of theinvention, the second junction layer is used as a hard mask for definingthe element.

According to embodiments of the first and second aspects of theinvention, the TiN in the first and second junction layers follow atleast one of the following process conditions:

-   -   a) deposition at about (100) degrees centigrade, and    -   b) deposition at about (200 Angstrom) in thickness.

The embodiments of the invention are useful for achieving a higherdensity of integration by using the Schottky diode that replaces theaddress transistor that is generally used for a reading operation fromthe memory cell. The Schottky diode has higher speed of operation thanthat of a p-n junction diode. Advantageously, the junction layer canalso be used as a hard mask for defining the magnetic storage element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a plan view of a section of an exemplary array ofmemory cells in a magnetic random access memory (MRAM) circuit.

FIG. 1B illustrates a partly schematic and partly cross-sectional viewof a memory cell described in FIG. 1A, with the memory cell including amagnetic tunnel junction (MTJ) stack.

FIG. 2A illustrates the view at FIG. 1B, with the region marked Ytherein being replaced by a Schottky diode that is serially connected tothe MTJ stack, according to an embodiment of the invention.

FIG. 2B is a fully schematic view of FIG. 2A, representing the MTJ stackwith a variable resistor.

FIG. 3A illustrates an etch process for defining the MTJ stack, using astack of a resist layer and the second junction layer as the maskinglayer.

FIG. 3B illustrates an etch process for defining the MTJ stack, usingthe second junction layer as the masking layer.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the invention. It will be apparent, however, to oneskilled in the art that the invention can be practiced without thesespecific details.

Reference in this specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment, nor are separate or alternative embodimentsmutually exclusive of other embodiments. Moreover, various features aredescribed which may be exhibited by some embodiments and not by others.Similarly, various requirements are described which may be requirementsfor some embodiments but not other embodiments.

Broadly, embodiments of the invention disclose an addressable memorycell where the normally used address transistor for the read operationis replaced by a Schottky diode. Embodiments for methods of operationand processing the memory cell are also disclosed. The embodiments areparticularly, though not exclusively, useful in forming an array inmagnetic random access memory (MRAM) circuits. The magnetic storageelement in the memory cell is for example, a magnetic tunnel junction(MTJ) stack. The diode being a uni-directional device, operates in ahigher current regime for the selected memory cells and in a lowercurrent regime for the unselected memory cells in the array.Advantageously, a high density memory array can be achieved with asufficient current differential between the selected and unselectedmemory cells. The diode being a Schottky diode formed between a metallayer and a junction layer, has a higher speed of operation than that ofa p-n junction diode. The junction layer can also be used as a hard maskfor defining the magnetic storage element, which provides processsimplicity and a higher accuracy of reproduction from a correspondingmask feature, than when defined with a resist masking layer. Accordingto an embodiment, the junction layer is a non-semiconductor layer likeTiN, which can be deposited at lower temperatures that can preventthermal degradation of an already formed MTJ stack.

Referring now to FIG. 2A of the drawings, there is shown a view of amemory cell 200A, that is similar to FIG. 1B but with the region markedY 134 therein being replaced by a Schottky diode 206 in series with theMTJ stack 202, according to an embodiment of the invention. Herein, theMTJ stack 202 is shown to have no extended bottom electrode 124 as inFIG. 1B, as it is redundant in this embodiment. The write WL 102 is indirect contact with the fixed layer 122, optionally through the firstvia hole 128. The Schottky diode 206 is formed between a junction layer204 and a metal layer (not shown) in BL 104. FIG. 2B is a fullyschematic view of FIG. 2A, representing the MTJ stack 202 as a variableresistor.

A write operation can be performed by energizing the BL 104 and the WL102 to generate a magnetic field for changing the magnetic state of theMTJ stack 202. For a read operation, a voltage is applied to the BL 104,so that a current can flow through the Schottky diode 206 and the MTJstack 202, to the WL 102. The magnitude of the current sensed indicatesthe conductivity and hence the magnetic state of the MTJ stack 202.

Advantageously, the junction layer 204 can also be used as a hard maskfor defining the MTJ stack 202 by etching. Use of hard masks in thefield of fabrication of integrated circuits are known to reduce processcomplexity and provide better reproduction of the mask feature on thesubstrate. FIG. 3A illustrates an etch process for defining the MTJstack 202, using another stack of a resist layer 302 and the junctionlayer 204 as the masking layer. As shown at step (i), the partiallyprocessed memory cell 300A has the resist layer 302 for defining theindividual junction layer 204 in the memory cell 300A. As shown at step(ii), subsequently, the individual MTJ stack 202 for the partiallyprocessed memory cell 300A is also defined by etching, without requiringa separate lithographic step. FIG. 3B illustrates a similar process fordefining the MTJ stack 202, using a masking layer of only the junctionlayer 204. As shown at step (iii), the partially processed memory cell300B has the resist layer 302 stripped after defining the individualjunction layer 204. As shown at step (iv), subsequently, the individualMTJ stack 202 for the partially processed memory cell 300B is defined byetching with only the junction layer 204 as the masking layer andwithout requiring a separate lithographic step.

In an alternate embodiment, the Schottky diode 206 can also be providedbetween the MTJ stack 202 and the WL 102, with suitable biasingconditions for operation. However, in this alternate embodiment, theadvantage of using the junction layer 204 as the hard mask for definingthe MTJ stack 202, is lost.

The drawings show the junction layer 204 as TiN. The embodiments of theinvention may equally use other suitable materials for forming theSchottky diode 206. The process conditions for the TiN are likely toneed optimization, for achieving the Schottky diode 206. According to anembodiment, the TiN has a thickness of about (200 Angstrom) and isdeposited at a temperature of about (100 C). Other thicknesses andtemperatures may equally be used with appropriate optimization. Theembodiments of the invention may also use magnetic storage elementsother than the MTJ stack 202. The metal layer in the BL 110 that formsthe Schottky diode 206 with the junction layer 204, may be aluminum orany other suitable metal. With the embodiment described, the memory cell200A is vertically stacked, which helps in denser integration and ineasier processing. The vertical stacking also enables stacking multiplememory cells 200A vertically within different metallization layers,thereby enabling higher integration density. The memory cell 200A may beused in an array in a magnetic random access memory (MRAM) circuit. TheMTJ stack 202 may have any kind of architecture and may use anycombination of materials as necessary, to meet the required performance.

The embodiments of the invention are compatible with any semiconductortechnology such as complementary metal-oxide-semiconductor (CMOS),bipolar-junction-transistor and CMOS (BiCMOS), silicon-on-insulator(SOI) and the like. The scope of the invention is also not limited toany particular technology in terms of processing sequence, materials,physical dimensions and the like.

The embodiments of the present invention may be applied to memorycircuits for applications in any area, such as in automotive, mobilephone, smart card, radiation hardened military applications, databasestorage, Radio Frequency Identification Device (RFID), MRAM elements infield-programmable gate array (FPGA) and the like.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be evident that the variousmodification and changes can be made to these embodiments withoutdeparting from the broader spirit of the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative senserather than in a restrictive sense.

1-20. (canceled)
 21. A memory cell, comprising: a first conductor; asecond conductor; a magnetic storage element; and a Schottky diodedisposed between the first conductor and the magnetic storage element,wherein the Schottky diode is configured to: enable reading of themagnetic storage element without activating an address transistorassociated with the memory cell; and provide a unidirectional conductivepath between the first conductor and the second conductor through themagnetic storage element.
 22. The memory cell of claim 21, wherein theSchottky diode comprises a junction layer disposed between the firstconductor and the magnetic storage element.
 23. The memory cell of claim21, wherein the Schottky diode comprises a titanium nitride (TiN)junction layer disposed between the first conductor and the magneticstorage element.
 24. The memory cell of claim 21, wherein the magnetstorage element comprises a magnetic tunnel junction (MTJ) stack. 25.The memory cell of claim 21, wherein the first conductor corresponds toa word line for the magnetic storage element and the second conductorcorresponds to a bit line for the magnetic storage element.
 26. Thememory cell of claim 21, wherein the first conductor corresponds to abit line for the magnetic storage element and the second conductorcorresponds to a word line for the magnetic storage element.
 27. Thememory cell of claim 21, wherein the first conductor, the magneticstorage element, the Schottky diode, and the second conductor arevertically stacked on a substrate.
 28. The memory cell of claim 21,wherein: the Schottky diode comprises a titanium nitride (TiN) junctionlayer disposed between the first conductor and the magnetic storageelement; and the TiN junction layer has a thickness of about 200Angstrom.
 29. A magnetic random access memory (MRAM) circuit,comprising: an array of addressable memory cells; and a plurality oflines configured to address memory cells of the array, wherein: eachmemory cell of the array includes a magnetic storage element and aSchottky diode disposed between the magnetic storage element and a firstline of the plurality of lines associated with the respective memorycell; the Schottky diode of each memory cell is configured to enablereading the magnetic storage element without activating an addresstransistor associated with the respective memory cell; and the Schottkydiode is further configured to provide a unidirectional conductive pathbetween the first line associated with the respective memory cell and asecond line associated with the respective memory cell.
 30. The MRAMcircuit of claim 29, wherein the Schottky diode of each memory cellcomprises a junction layer disposed between its respective first lineand magnetic storage element.
 31. The MRAM circuit of claim 29, whereinthe Schottky diode of each memory cell comprises a titanium nitride(TiN) junction layer disposed between its respective first line andmagnetic storage element.
 32. The MRAM circuit of claim 29, wherein themagnet storage element of each memory cell comprises a magnetic tunneljunction (MTJ) stack.
 33. The MRAM circuit of claim 29, wherein thefirst line associated with each memory cell corresponds to a word linefor the respective memory cell and the second line associated with eachmemory cell corresponds to a bit line for the respective memory cell.34. The MRAM circuit of claim 29, wherein the first line associated witheach memory cell corresponds to a bit line for the respective memorycell and the second line associated with each memory cell corresponds toa word line for the respective memory cell.
 35. The MRAM circuit ofclaim 29, wherein the first line, the magnetic storage element, theSchottky diode, and the second line of each memory cell are verticallystacked on a substrate.
 36. The MRAM circuit of claims 29, wherein: theSchottky diode of each memory cell comprises a titanium nitride (TiN)junction layer disposed between its first line and magnetic storageelement; and the TiN junction layer of each memory cell has a thicknessof about 200 Angstrom.
 37. A method of fabricating a magnetic memorycell, comprising: forming a first line of the magnetic memory cell;forming, on the first line, a plurality of layers for a magnetic storageelement; forming a junction layer of the Schottky diode on the pluralityof layers; forming a masking layer on the junction layer; etching thejunction layer based on the masking layer to form the Schottky diode;stripping the masking layer from the junction layer; etching theplurality of layers using the stripped junction layer as another maskinglayer to form the magnetic storage element; and forming a second line ofthe magnetic memory element on the stripped junction layer.
 38. Themethod of claim 37, wherein said forming a plurality of layerscomprises: forming a free layer of the magnetic storage element on thefirst line; forming a tunnel oxide layer of the magnetic storage elementon the free layer; and forming a fixed layer of the magnetic storageelement on the tunnel oxide layer.
 39. A method of claim 37, whereinsaid forming a junction layer comprises depositing a TiN layer for thejunction layer at about 100 degrees centigrade.
 40. A method of claim37, wherein said forming a junction layer comprises depositing a TiNlayer for the junction layers at about 200 Angstrom in thickness.